1. Field of the Invention
This invention relates to the field of crystalline materials to be used in fabrication of the core and the cladding of diffusion bonded optics. More particularly, it pertains to the use of undoped yttrium aluminum garnet (hereinafter, YAG) for the fabrication of the cladding, and of doped lutetium aluminum garnet (hereinafter, LuAG) for the fabrication of the core, the core being a rod completely surrounded by the cladding on all sides including the ends of the rod.
2. Description of the Related Art
Many modern waveguided structures comprise a core diffusion-bonded to cladding in which this core is completely ensconced (except for the ends). Prior art for such structures incorporated sapphire as the cladding material. For example, U.S. Pat. No. 5,852,622 to Meissner et. al., teaches a system for improving the thermal uniformity of composite solid state lasers which system also gives these lasers some other advantageous properties. Some embodiments of Meissner's invention use sapphire as the cladding material.
For high-brightness, high-power laser systems, a traditionally preferred approach, in order to maintain a good beam quality, is use of a phase-conjugate master oscillator power amplifier (MOPA) laser architecture. For maximum flexibility in power scaling and pulse format, guided wave amplifiers in a loop architecture phase conjugotor based on thermal non-lineary interaction have been previously chosen.
In specific examples of prior art, the ytterbium-doped YAG core was used as the gain medium. Sapphire cladding was used to establish guiding and to achieve thermal robustness. See, e.g., Sumida, et. al. “Diode-Pumped Yb:YAG Catches Up With Nd:YAG,” Laser Focus World, June 1999.
While the design employing the ytterbium-doped YAG core and sapphire cladding is a viable design having some advantages and positive sides, it also has several problems and drawbacks. Both advantages and disadvantages of such prior art design are discussed hereinbelow.
Laser guiding, via total internal reflection (TIR), takes place when the refractive index of the core is larger than that of the cladding. TIR occurs when the angle of incidence is larger than the critical angle θcritical, where θcritical is determined from the equation in θcritical=ncladding/ncore, where ncladding and ncore are refractive indices of the cladding and the core, respectively.
It follows from the foregoing that the angular spread of what is guided depends on the difference in the refractive indices, and the advantages of sapphire include its much lower refractive index relative to YAG (1.76 and 1.82, respectively).
In addition, sapphire has superior thermal conductivity compared to that of YAG (about 35 Watts per meter per Kelvin for sapphire compared to 10 Watts per meter per Kelvin for YAG), which is important to facilitate the removal of heat from the YAG core.
However, this prior art design employing sapphire had serious disadvantages, particularly in that it caused difficulties with the six sided bonding using sapphire. The dissimilar crystalline and mechanical properties make the practical fabrication of diffusion-bonded fully encased sapphire and YAG composite structures very difficult. Those skilled in the art are aware that the manufacturing of diffusion-bonded ytterbium-doped YAG-sapphire composites, promoted by onyx Optics, Inc. of Dublin, Calif., albeit successful, has proved to be very time-consuming as well as very expensive undertaking.
Specifically, because the polishing rate for the two materials is different, it is very difficult to obtain an adequately flat optical finish over a sapphire-YAG-sapphire composite surface.
Another drawback of the sapphire-based technology is that according to this technology it is essential to obtain surface free of scratches, gaps, voids, inclusions, digs and similar mechanical imperfections in preparing the surface for any subsequent bonding steps. Although this disadvantage may characterize any known diffusion bonding technique, in the case of YAG it is mitigated because it is easier to polish YAG than to polish sapphire.
Also, the crystalline axis of the sapphire (a uniaxial crystal) must be oriented in a particular direction relative to the YAG interface for reliable diffusion bonding. A simple geometrical analysis shows that this preferred orientation is not possible for bonding on all six sides. Hence in practice, a YAG core fully encased in sapphire cladding is very difficult to manufacture.
In view of the foregoing problems and disadvantages inherent in composite structures having sapphire cladding, there exists, therefore, a need for an improved waveguided structure. Such structure should comprise a core encased in cladding, and diffusion-bonded thereto, allowing to use more economical bonding, where a solid endcap is easily bonded to the composite core-cladded endface. The structure should also have acceptable entendue, provide better extraction of energy from the device and be more thermally robust.
There exists no known prior art describing a waveguided structure having all the advantages and benefits described above. Yet the need for such is acute. The present invention discloses such structure and method of fabrication thereof.